An optical disc, such as a compact disc (CD), is an optical data storage medium that data can be written to and read using a low-powered laser beam. Optical disc technology first appeared in the marketplace with the CD, which is typically used for electronically recording, storing, and playing back audio, video, text, and other information in digital form. A digital versatile disc (DVD) is another more recent type of optical disc that is generally used for storing and playing back movies because of its ability to store much more data in the same amount of physical space as a CD.
Compact discs were initially a read-only storage medium that stored digital data as a pattern of depressions and flat areas impressed into a piece of clear polycarbonate plastic through a complex manufacturing process. However, average consumers can now write data onto their own CDs with CD players capable of writing digital data onto CD-Rs (CD-recordable discs), CD-RWs (CD-rewritable discs), and the many other forms of optical discs.
Methods for labeling the non-data side of such optical discs with text and images, for example, have continued to develop as consumers desire more convenient ways to identify their own recorded discs. Basic methods for labeling a disc include physically writing on the non-data side with a permanent marker (e.g., a Sharpie marker), or printing out a paper sticker label and sticking it onto the non-data side of the disc. Other physical marking methods developed for implementation in conventional optical disc players include ink jet printing, thermal wax transfer, and thermal dye transfer methods. Still other methods use the laser in a conventional disc player to mark a specially prepared disc surface.
A label image can be rendered on the label surface (i.e., the non-data side, or top side) of an optical disc by marking the label surface with a laser beam along concentric circular tracks around the disc. When an optical disc is labeled on concentric circular tracks, however, unmarked spaces (e.g., a white space) between the start and the end of the tracks may appear as a light colored radial stripe within a label image. A space between the start and the end of a track is typically a fraction of a marking space and is therefore not marked with the laser because an entire mark will not fit into the fractional marking space. These unmarked fractional spaces are commonly referred to as end-of-track gaps and are visually objectionable.
Alternatively, if the unmarked fractional spaces at the end of the circular tracks are marked, or written over, a dark colored radial stripe may appear within light or mid-tone regions of a label image which is also visually objectionable. The dark stripe will appear because a mark that is written into a fractional marking space will overlap the first mark of the track (or the last mark of the track, or both) and the overlapped fraction of the first mark may appear darker than the other marks. Overlapping marks are commonly referred to as end-of-track overwrites.
FIG. 1 illustrates the end-of-track gaps and overwrite problems associated with labeling, or marking, an optical disc 100. The disc 100 includes a disc label region 102 and a text label 104 that has been written on the disc 100 in the label region 102. A region 106 of the text label 104 is expanded to illustrate that unmarked spaces between the start 108 and the end 110 of tracks 112 will appear as a light colored radial stripe 114 within a label image, such as within the letter “x” of the text label 104. The unmarked fractional spaces between the start 108 and the end 110 of the tracks 112 vary as a fraction of a marking space in this example.
The tracks 112 in the expanded region 106 includes multiple laser marks 116 which are written onto disc 100 with a laser to form the expanded region 106 of the letter “x”. The marks 116 are written to form the tracks 112 which are concentric circular tracks around the disc 100 in a direction indicated by arrow 118 (i.e., each concentric circular track 112 starts at the position 108 and ends approximately at the position 110). The tracks 112 may also be written in an opposite direction to that which is indicated by arrow 118 by some conventional labeling systems. A track 112(N) illustrates an end-of-track overwrite where a laser mark 120, written at the end of track 112(N), overlaps a first laser mark 122 creating a darker image than is desired when writing the image on the disc 100.
A gap 114 between laser marks generally occurs at the end of a track because the track length (e.g., the circumference of the track) is not an integer number of mark spaces in length. For example, the space 114 is approximately a one-half fraction of a mark 116 as shown in FIG. 1. The track lengths are not integral multiples of the mark spacing because the radius of an inner track 124 is typically selected to match a nominal print region inner radius 126. Since the circumference of this inner track 124 is a product of two-pi and the radius 126 (i.e., C=2π·R), the circumference of the track 124 will generally not be an exact integer multiple of the mark spacing.
Further, the spacing between the concentric circular tracks 112 in a radial direction 126 is typically selected as a simple ratio of the mark spacing when writing a label image onto an optical disc. For example, if the shape of the marks created with the laser are circular, selecting a track spacing that is equivalent to the mark spacing will result in a uniform print density both radially and tangentially. If the shape of the marks are ellipsoidal, the track to mark spacing ratio can be adjusted to achieve a uniform print density of the label image. However, when simply selecting a first track radius 126 that corresponds to an inner track 124 of the label region 102, and/or when selecting a track spacing to account for a uniform print density only, end-of-track gaps 114 and/or overwrites will appear in the label image because the track lengths will not be integral multiples of the mark spacing.
Accordingly, a technique is needed to visually enhance disc media marking, such as optical disc labeling, and to avoid end-of-track gaps and/or overwrites of laser marks.